1. Field of the Invention
The invention relates to a twin screw extruder comprising a casing with a working direction; two partially intersecting casing bores, which are parallel to each other; two shafts, which are disposed in the casing bores and which are drivable to rotate in the same direction of rotation about an axis of rotation, the axes of rotation having a distance A from each other; and single-flight kneading disks, which are fixed on the shafts and have surface lines extending parallel to the respective axis of rotation, the kneading disks, in a cross-section perpendicular to the axis of rotation, comprising a crest, which is formed as a segment of a circle about the respective axis of rotation and which has a crest angle b and a radius R.sub.A ; a bottom, which is formed as a segment of a circle about the respective axis of rotation and has a bottom angle g and a radius R.sub.I ; two flanks, which join the crest and the bottom; R.sub.A &gt;R.sub.I und A.apprxeq.R.sub.A +R.sub.I applying.
2. Background Art
Single-flight kneading disks are known from DE 813 154 B. They have a crest angle greater than 90.degree. , in this regard possessing a comparatively important cross-sectional surface. The kneading disks wipe the casing as well as themselves. In these known kneading disk arrangements, no overall conveyance of the treated material takes place in or against the operating direction of an extruder which lodges the kneading disks. The mixing effect by this type of kneading disks is low. The same is true for the kneading effect. Furthermore, also the free cross-sectional surface is defined and thus the mean dwell time, within a lengthwise section of the extruder, of the material to be treated.
U.S. Pat. No. 5,573,332 teaches a screw element for a screw-type extrusion machine. The screw elements are helical and have varying pitch directions. Lengthwise mixing is obtained by the screwing in opposite directions, whereas crosswise mixing is attained by the elongated wedge of the flank arc. This crosswise flow is a typical continuous shear flow, which is primarily a dispersive mixing operation. Dividing the flow into various partial flows, recirculation and offset combination thereof do not take place, which is why the distributive mix is not optimal.
DE 43 38 795 C teaches a continuously working multi-screw extruding machine for masses to be plastified. Provided between two closely intermeshing conveying screws and a dam-up element is a screw which conveys forwards and adjacent thereto a non-intermeshing screw which conveys backwards. A drawback resides in that the wall of the casing cannot be scraped off, since the screws have a diameter that corresponds to half the center distance. This does not prevent the material to be treated from sticking on the casing wall. Since the casing wall is not wiped, this results in very bad heat transmission. In the case of high differences of temperature between the casing wall and the product, this will result in important inhomogeneities of temperature which will moreover negatively affect the material to be treated.